Kosmochlor-bearing Omphacitite From West Sayan Jadeite Jade Area

Omphacitite that catches lots of scholars interest and attention is a relatively rare rock that occasionally exposed in jadeitite area and UHP metamorphic zone. In the jadeitite area, Myanmar is the most important place in that the omphacitite was reported. And the omphacitite is thought to be a index of multi-stage jadeitizations. Omphacitite was found in the research of West Sayan jadeitite. The omphacitite mainly contains omphacite and kosmochlor, also contains chromite, phlogopite and a small amount of pentlandite, and the content of omphacite reaches more than 90%. There are mainly three distinct textures in the omphacitite:corona texture, subgrain. The omphacitite shows the evolution of corona texture from the development to the full formwith the containing of Cr in the kosmochlors in the core of corona texture increasing. The chemical composition and the texture show two or more stages of jadeitization in West Sayan omphacitite:(1) the main period of filling metasomatism. In this stage, The kosmochlor forms by the reaction between jadeitic fluids and chromite, and the omphacite forms by the interaction of pyroxene and jadeitic fluid; (2) the period of filling metasomatism related to metamorphism. In this stage, the series of new Ae-Di-Ko sosoloid formed. And the jadeitization controls the desalinization of chromium that is divided into four stages:(1) transformation from chromite to kosmochlor; (2) from kosmochlor to high chromium-bearing omphacite; (3) from high chromium-bearing omphacite to low chromium-bearing omphacite, and (4) from chromium-bearing omphacite to lower chromium-bearing omphacite, which is essentially the further evolution of the stage 3. Using the desalinization of chromium, the feature that green colour in jadeite jade has special shape can be interpreted, and how and what the green colour changes can be understood.The comparison between West Sayan omphacitite and Myanmar omphacitite shows: West Sayan omphacitite contains kosmochlor, but lack of jadeite, and there are corona textures related to kosmochlor. In chemical compositions, West Sayan omphacitite contains more Mg and Ca, less Fe and lack of k, whereas Myanmar omphacitite contains a little of K. To resolve the origin of jeditic fluid/melt that form the jadeitite and the omphacitite is very important for a reasonable interpretation of the genesis of jadeitite.Recent researchs shows the jadeitic fluid is more likely related to sediments of subducted slabs. According to recent researchs, the jadeitite thought to be the the production of the crystallization of jadeitic fluid.As the process of oceanic crust subducting to the continental plate, the sediments of subducted slabs dehydrate or melt with the temperature and pressure increasing, and the melt uplift,then the crystallization of the melt under certain conditions forms the jeditite. The omphacitite forms by the interaction of pyroxenite rich in Fe and Ca in ophiolite complex and jadeite fluids at HP/LT conditions during active subduction/collision. The kosmochlor forms by the reaction between jadeite fluids and chromite at the same time of the forming of omphacitite.